Problem: A cylindrical water tank is $\frac{1}{5}$ full. If three liters were added, the tank would be $\frac{1}{4}$ full. How many liters does the tank hold when it is full?
Let the number of liters of water in the tank originally be $w$, and let the number of liters of water the tank can hold when it is full be $c$. Originally, we have the equation $\frac{w}{c}=\frac{1}{5}$. Cross multiplying, we have $c = 5w$, or $w=\frac{c}{5}$. After three liters of water are added, we have the equation $\frac{w+3}{c} = \frac{1}{4}$. Cross multiplying, we have $c=4w+12$. Substituting the previous expression for $w$ into this last equation to eliminate $w$, we get $c=4(\frac{c}{5})+12$, or $c=60$. Thus, the number of liters of water the tank can hold is $\boxed{60}$.